Oceanography Vol. 20, No. 130

Challenger DeepUnusual life forms may have evolved under conditions of extreme pressure in this 11,000-meter-deep trench, the deepest part of the world’s oceans.

New ZealandThis region has a full spectrum of habitats supporting seafl oor life (hydrothermal vents, cold seeps, whale carcasses, and wood from shipwrecks and trees) in close proximity. How have species evolved in these diverse settings?

Chile RiseThis region has a variety of chemosynthetic habitats and geological features in close prox-imity. How do seafl oor popula-tions diverge or converge at this triple junction on the “highway” of mid-ocean ridges?

Soon after animal communities were discovered around seafl oor hydrothermal vents in 1977, sci-entists found that vents in various regions are populated by distinct animal species. Scien-tists have been sorting clues to explain how seafl oor populations are related and how they evolved and diverged over Earth’s history. Scientists today recognize dis-tinct assemblages of animal species in six major seafl oor regions (colored dots) along the system of volcanic mountains and deep-sea trenches that form the borders of Earth’s tectonic plates. But unexplored ocean regions remain critical missing pieces for assembling the full evolutionary puzzle.

MissingPieces

E. Paul Oberlander

On the Sea�oor, Di�erent Species Thrive in Di�erent Regions

South AtlanticPowerful currents and huge seafl oor chasms (fracture zones) may act as barriers blocking the dispersal of vent larvae and dis-connecting vent populations in the North and South Atlantic.

CaribbeanIn this region, methane seeping from the seafl oor also supports animal communities. Did animals migrate between “cold seeps” and nearby hot vents over evolutionary history?

Arctic OceanThe Arctic Ocean has never had deep connections with other major oceans. It may harbor fundamentally different vent animals that evolved in isolation over the past 25 million years.

Southern OceanThe Drake Passage may act as a key link or bottleneck for larval dispersal between the Atlantic and Pacifi c. Whale carcasses and shipwrecks (such as Shackleton’s Endurance) may offer refuges or stepping-stones between vents.

•Central Indian vent com-munities are populated by

Western Pacifi c-type fauna, but also have North Atlantic-type shrimp species.•Northeast Pacifi c vent

communities are domi-nated by “bushes” of skinny tube-worms called Ridgea piscesae.

•Western Pacifi c vent com-munities are dominated by

barnacles and limpets, as well as hairy gastropods, shown above.

•Shallow Atlantic vents (800-1700-meter depths)

support dense clusters of mussels on black smoker chimneys.

•Eastern Pacifi c vent com-munities are dominated

by tall, fat tubeworms calledRiftia pachyptila.

•Deep Atlantic vent com-munities (2500-3650-me-

ter depths) are dominated by swarms of shrimp called Rimica-ris exoculata.

The discovery of hydrothermal vents and the unique, often endem-

ic fauna that inhabit them represents one of the most extraordinary

scientific discoveries of the latter twentieth century. Not surprising-

ly, after just 30 years of study of these remarkable—and extremely

remote—systems, advances in understanding the animals and mi-

crobial communities living around hydrothermal vents seem to

occur with every fresh expedition to the seafloor. On average, two

new species are described each month—a rate of discovery that has

been sustained over the past 25–30 years (Van Dover et al., 2002;

Fisher et al., this issue). Furthermore, the physical, geological, and

geochemical features of each part of the ridge system and its asso-

ciated hydrothermal-vent structures appear to dictate which nov-

el biological species can live where. Only 10 percent of the ridge

system has been explored for hydrothermal activity to date (Baker

and German, 2004), yet we find different diversity patterns in that

small fraction. While it is well known that species composition var-

ies along discrete segments of the global ridge system, this “bio-

geographic puzzle” has more pieces missing than pieces in place

(Figure 1, Table 1).

In this paper, we start with a general picture of the global ridge

system and hydrothermal environments, then continue with a de-

scription of known biodiversity—including physiological adapta-

tions—and species distribution, which leads into a discussion of

the factors that might drive observed biogeography patterns. We

conclude with a look toward the future, describing the main efforts

being made to fill the gaps in our knowledge of vent biogeography.

S p e c i a l i S S u e F e at u r e

Thirty Years of Discovery and investigations

B Y e V a r a m i r e z- l l o D r a ,

t i m o t h Y m . S h a N k , a N D

c h r i S t o p h e r r . G e r m a N

Oceanography Vol. 20, No. 130

Biodiversity and Biogeographyof hydrothermal Vent Species

This article has been published in Oceanography, Volume 20, Number 1, a quarterly journal of The oceanography Soci-ety. copyright 2007 by The oceanography Society. all rights reserved. permission is granted to copy this article for use in teaching and research. republication, systemmatic reproduction, or collective redistirbution of any portion of this article by photocopy machine, reposting, or other means is permitted only with the approval of The oceanography Society. Send all correspondence to: info@tos.org or Th e oceanography Society, po Box 1931, rockville, mD 20849-1931, uSa.

Oceanography march 2007 31

Challenger DeepUnusual life forms may have evolved under conditions of extreme pressure in this 11,000-meter-deep trench, the deepest part of the world’s oceans.

New ZealandThis region has a full spectrum of habitats supporting seafl oor life (hydrothermal vents, cold seeps, whale carcasses, and wood from shipwrecks and trees) in close proximity. How have species evolved in these diverse settings?

Chile RiseThis region has a variety of chemosynthetic habitats and geological features in close prox-imity. How do seafl oor popula-tions diverge or converge at this triple junction on the “highway” of mid-ocean ridges?

Soon after animal communities were discovered around seafl oor hydrothermal vents in 1977, sci-entists found that vents in various regions are populated by distinct animal species. Scien-tists have been sorting clues to explain how seafl oor populations are related and how they evolved and diverged over Earth’s history. Scientists today recognize dis-tinct assemblages of animal species in six major seafl oor regions (colored dots) along the system of volcanic mountains and deep-sea trenches that form the borders of Earth’s tectonic plates. But unexplored ocean regions remain critical missing pieces for assembling the full evolutionary puzzle.

MissingPieces

E. Paul Oberlander

On the Sea�oor, Di�erent Species Thrive in Di�erent Regions

South AtlanticPowerful currents and huge seafl oor chasms (fracture zones) may act as barriers blocking the dispersal of vent larvae and dis-connecting vent populations in the North and South Atlantic.

CaribbeanIn this region, methane seeping from the seafl oor also supports animal communities. Did animals migrate between “cold seeps” and nearby hot vents over evolutionary history?

Arctic OceanThe Arctic Ocean has never had deep connections with other major oceans. It may harbor fundamentally different vent animals that evolved in isolation over the past 25 million years.

Southern OceanThe Drake Passage may act as a key link or bottleneck for larval dispersal between the Atlantic and Pacifi c. Whale carcasses and shipwrecks (such as Shackleton’s Endurance) may offer refuges or stepping-stones between vents.

•Central Indian vent com-munities are populated by

Western Pacifi c-type fauna, but also have North Atlantic-type shrimp species.•Northeast Pacifi c vent

communities are domi-nated by “bushes” of skinny tube-worms called Ridgea piscesae.

•Western Pacifi c vent com-munities are dominated by

barnacles and limpets, as well as hairy gastropods, shown above.

•Shallow Atlantic vents (800-1700-meter depths)

support dense clusters of mussels on black smoker chimneys.

•Eastern Pacifi c vent com-munities are dominated

by tall, fat tubeworms calledRiftia pachyptila.

•Deep Atlantic vent com-munities (2500-3650-me-

ter depths) are dominated by swarms of shrimp called Rimica-ris exoculata.

Figure 1. Schematic illustration of the global mid-ocean ridge system. Scientists today recognize distinct assemblages of animal species in six major seafloor regions (colored dots) along the system of volca-nic mountains and deep-sea trenches that form the borders of earth’s tectonic plates. however, unexplored ocean regions remain critical missing pieces for assembling the full evolutionary puzzle. From Shank (2004)

Oceanography march 2007 31

Oceanography Vol. 20, No. 132

aBiotic Setup oF the VeNt haBitat: phYSical , GeoloGical , aND Geochemical propertieSToday, satellite technology provides a

clear visualization of the globe-encircling

mid-ocean ridge. It is here, at seafloor-

spreading centers, that lava wells up from

Earth’s interior to generate fresh oce-

anic crust (see Langmuir and Forsyth,

this issue). While a small proportion

of these volcanic systems are found in

back-arc basins close to subduction

zones (see Martinez et al., this issue), the

vast majority of all Earth’s mid-ocean

ridges form a single, continuous, globe-

encircling volcanic chain that is roughly

60,000 km in length. Mid-ocean ridges

typically lie at around 2000- to 5000-m

depth and therefore represent environ-

ments that experience high pressures,

complete darkness, and ambient tem-

peratures of only ~ 2°C.

table 1. Dominant fauna of main proposed biogeographical provinces.

Biogeographical Province and Depth Dominating Fauna

azores (shallow north atlantic, 800-1700 m) Bathymodiolid mussels, amphipods, and caridean shrimp

mid-atlantic ridge between azores triple Junction and equator (deep north atlantic 2500-3650 m)

caridean shrimp—mainly Rimicaris exoculata—and bathymodiolid mussels

South mid-atlantic ridge caridean shrimp, bathymodiolid mussels, and clams

east pacific rise and Galápagos rift Vestimentiferan tubeworms—mainly Riftia pachyptila—and bathymodiolid mussels, vesicomyid clams, alvinellid polychaetes, amphipods, and crabs

Northeast pacific Vestimentiferan tubeworms excluding riftiidae, polychaetes, and gastropods

Western pacific Barnacles, limpets, bathymodiolid mussels, “hairy” gastropod, vesicomyid clams, and shrimp

central indian ridge caridean shrimp Rimicaris kairei, and mussels, “scaly” gastropods, and anemones

Spreading rates for mid-ocean

ridges—the speed at which two tec-

tonic plates are being pulled apart from

each other—vary across the whole sys-

tem. Ridges can be classified accord-

ing to their spreading rates into ultra-

slow spreading (< 20 mm yr-1) such

as the Arctic Gakkel Ridge (see Snow

and Edmonds, this issue), slow spread-

ing (20–50 mm yr-1) such as the Mid-

Atlantic Ridge, intermediate spreading

(50–90 mm yr-1) such as the Central

Indian Ridge, and fast (90–130 mm yr-1)

to superfast (130–170 mm yr-1) spread-

ing such as the East Pacific Rise. Spread-

ing rates are important in shaping ridge

morphology (MacDonald et al., 1991)

and, as a consequence, the hydrodynam-

ics of the habitat. At slow-spreading

rates, the walls of the ridge axis are

separated by a deep (1–3 km) and wide

(5–15 km) rift valley that constrains

dispersing hydrothermal plumes. Some

of the largest and longest-lived hydro-

thermal vents discovered thus far occur

along the slow-spreading Mid-Atlantic

Ridge. In contrast, fast-spreading cen-

ters exhibit shallow and narrow (order

of tens of meters) summit calderas

where hydrothermal plumes are not

constrained. Along the fastest sections

of the East Pacific Rise, vent sites are so

geographically close that they can form

a continuum along the ridge axis. These

topographic and physical characteristics

have the potential to affect the patterns

of faunal distribution (see below). The

western Pacific Ocean seafloor is char-

acterized by a complex system of back-

arc basins and volcanic arcs with active

hydrothermal venting. These back-arc

basins are isolated spreading centers that

have been active for less than 10 million

years—a short geological time com-

pared to the ages of mid-ocean ridges

(Hessler and Lonsdale, 1991). Their iso-

Oceanography march 2007 33

eVa ramirez-lloDra (ezr@icm.csic.es)

is Research Scientist, National Ocean-

ography Centre, Southampton, UK, and

Institut de Ciències del Mar (CMIMA-CSIC),

Barcelona, Spain. timothY m. ShaNk

is Associate Scientist, Biology Department,

Woods Hole Oceanographic Institution,

Woods Hole, MA, USA. chriStopher

r. GermaN is Senior Scientist and Chief

Scientist for Deep Submergence, Geol-

ogy and Geophysics Department, Woods

Hole Oceanographic Institution, Woods

Hole, MA, USA.

lation—both from one another and from

mid-ocean ridges—and their relatively

short active geological ages make back

arcs particularly interesting habitats for

biogeographic and gene-flow analyses

(Desbruyères et al., 2007; see below).

Hydrothermal circulation occurs at

mid-ocean ridges when dense, cold sea-

water percolates downward through frac-

tured oceanic crust near the ridge crest

(Figure 2). The vent fluid is geothermally

heated close to the magma chamber that

feeds the ridge, reaching temperatures

that can exceed 400°C. The fluid is also

chemically modified, losing all dissolved

oxygen and accumulating high con-

centrations of dissolved reduced gases

such as methane and hydrogen sulfide).

They are also strongly acidic (pH 2–3)

compared to the near-neutral character

(~ pH 8) of the deep oceans and are rich

in numerous metals—dominantly iron,

manganese, copper, and zinc, but also

precious metals such as silver, gold, and

platinum, as well as those often thought

of as highly toxic to life (e.g., cadmium,

mercury, arsenic and lead). The buoyant,

superheated fluid is expelled into the wa-

ter column through polymetallic sulfide

chimneys or “black smokers” that can

tower tens of meters above the seafloor

(Figure 3). While approximately half of

all hydrothermal fluids are currently be-

lieved to be discharged from the seafloor

in this spectacular black-smoker form,

about an equal amount mixes with cold

seawater and percolates down into the

highly fractured seafloor as more dilute

warm waters. The latter have precipitated

their mineral load but, importantly, still

contain high concentrations of dissolved

methane and hydrogen sulfide, which are

emitted from the seabed in this “diffuse-

flow” form, typically at temperatures that

are measured in tens of degrees Celsius

(further detailed information on hydro-

thermal vent chemistry and functioning,

and associated mineral deposits, can be

found in an article by Tivey in this issue).

It is these vent fluids that provide the

necessary energy, in the form of reduced

chemicals, for the development of the

lush faunal communities found at vents.

The hydrothermal trophic web is based

in the production of chemoautotrophic

Figure 2. Schematic illustra-tion of hydrothermal fluid cir-

culation beneath the seafloor at mid-ocean ridge crests. From

German and Von Damm (2004)

Oceanography Vol. 20, No. 134

Figure 3. Black smoker from the North mid-atlantic ridge. Photo credit:

Deutsche-Ridge Schwerpunkt Program and the MARUM-Quest ROV

microbes (Cavanaugh, 1983; Jannasch,

1984) that use reduced compounds from

the vent fluids to fix carbon dioxide

from seawater. The microbes are found

free-living as well as in highly success-

ful symbiosis with many of the macro-

invertebrates inhabiting the vent habitat

and they are responsible for the very high

production at hydrothermal vents.

BioGeoGr aphY oF hYDrothermal VeNtS: a kNoWleDGe puzzle uNDer coNStructioNBiodiversity and adaptationsA sound knowledge of an ecosystem’s

species composition is essential for any

subsequent biogeographic investigations

and to understand the factors driving

distribution patterns. The dawn of bio-

diversity studies in hydrothermal vents

takes us back to nine black-and-white

photographs of large white bivalves on

black basalt, taken by the Deep-Tow

Camera System on May 29, 1976, on the

Galápagos Spreading Center (Lonsdale,

1977). A year later, during Alvin dives re-

visiting the region, geologist Jack Corliss

gave astonishing accounts of the first

observations of giant tubeworms later

described as Riftia pachyptila (Corliss et

al., 1979) and thick beds of mussels as-

sembled around seafloor openings emit-

ting diffuse vent fluids. Only 30 years

have passed since those initial discover-

ies; we now know of more than 550 vent

species and their composition and dis-

tribution at more than 100 vent sites

along the global mid-ocean ridge system

(Desbruyères et al., 2006).

The first impression when looking at

a vent field is the profusion of life, with

high densities and biomass of exotic ani-

mals (Figure 4). This bounty explains

why hydrothermal vents are often re-

ferred to as “oases of the deep.” While

biomass is extremely high relative to

the surrounding deep sea, biodiversity

at vents is low, especially in relation to

the high biodiversity of non-chemosyn-

thetic, deep-sea benthos (Grassle and

Macioleck, 1992). This relationship is

true for all groups, from meio- and mac-

rofauna found on oceanic basalt and sed-

iments highly modified by hydrothermal

fluid to macro- and megafauna living

on mussel beds or tubeworm fields (Van

Dover, 2000). The low diversity and high

densities of individuals in vent commu-

nities is typical of habitats with high en-

ergy availability and environmental con-

ditions with high or low values within

their range, such as very wide tempera-

ture ranges or high levels of toxic chemi-

cals. The most speciose phyla found at

hydrothermal vents are the arthropods,

followed by the molluscs and the anne-

lids. However, our knowledge of the di-

versity and distribution of vent species is

still limited to an extremely small section

of the global ridge system, and every new

detailed survey and investigation, both

on newly discovered sites and on well-

explored ones, brings new species and

ecologies to be identified and described,

as well as new insight to the phylogeog-

raphy of vent species.

One major characteristic of vent bio-

logical communities is the high degree

(~ 85 percent) of species endemism,

with many species showing important

physiological, morphological, and eco-

logical adaptations to particular environ-

Oceanography march 2007 35

mental factors. Arguably, one of the most

remarkable vent animals is the giant

tubeworm Riftia pachyptila from the

Galápagos Rift and East Pacific Rise. This

worm and related species were first de-

scribed as a new phylum, Vestimentifera,

but have now been moved to the family

Siboglinidae in the Phylum Annelida,

Class Polychaeta (Halanych, 2005). Riftia

pachyptila lack a mouth and a diges-

tive system, and depend completely for

growth and reproduction on organic

matter produced by their endosymbiotic

chemosynthetic bacteria. These bacteria

are found densely packed in an organ

that spans the interior of the animal’s

body, called the trophosome. The tube-

worm absorbs hydrogen sulfide, carbon

dioxide, and oxygen through its well-

irrigated plume and transports them to

the trophosome via the vascular system

along with modified hemoglobin, which

transports unbound or available sulfide.

A different fascinating polychaete

from the East Pacific Rise is the Pompeii

worm, Alvinella pompejana. This poly-

chaete lives inside organic tubes that

it builds on the exterior walls of active

chimneys in close proximity to venting

fluid; in contrast to the Riftia habitat,

the chemistry of this fluid is domi-

nated by iron-bound complexes with

hydrogen sulfide (Luther et al., 2001).

The maximum temperature tolerated

by this species is a continuing subject

of debate, with values ranging between

10°C and 80°C (Gaill and Hunt, 1991;

Cary et al., 1998).

In the crustaceans, a remarkable ex-

ample of sensory adaptation is found on

the caridean shrimp Rimicaris exoculata

from the Mid-Atlantic Ridge. These

shrimp live in massive, dense, and dy-

namic aggregations (3000 per liter;

Gebruk et al., 2000) on the chimney

walls, feeding on symbiotic bacteria cul-

tivated within their enlarged gill cham-

bers, on their external carapace, and on

ingested mineral particles. The “eyes”

of R. exoculata are considered to have

evolved into a broad ocular plate that

forms part of the shrimp’s dorsal sur-

face. The plate’s novel photoreceptors

apparently do not form a distinct image;

instead, these “eyes” are highly sensitive

to dim light, perhaps an adaptation for

detecting radiation from the orifices of

hot black smokers and for allowing the

shrimp to orient to the chimney walls

(Van Dover et al., 1989).

Examples of metabolic, morphologi-

cal, sensory, and symbiotic adaptations

are abundant at the vent habitat, and ex-

ploration of new sites still results in the

discovery of striking animals, such as the

“scaly foot gastropod” recently found on

Figure 4. The giant tubeworm Riftia pachyptila, the mussel Bathymodiolus, and crabs from east pacific rise vents. Image courtesy of Richard A. Lutz

Oceanography Vol. 20, No. 136

Figure 5. The “scaly foot gastropod” from the indian ocean ridge. Image courtesy of Charles Fisher

the Central Indian Ridge (Figure 5). This

gastropod, still awaiting description, har-

bors thiotrophic (sulfurous) bacteria in

a greatly enlarged esophageal gland and

has an operculum modified into several

hundred aligned scales covered by thick

layers of iron sulfides, whose purpose is

unknown (Warén et al., 2003).

known Distribution patterns of Vent SpeciesAlthough we have only explored a

very limited proportion of the active

hydrothermal vents that potentially exist

worldwide, the description of the fauna

from the vent sites that we do know is

already sufficient to demonstrate that

different faunal communities character-

ize different ocean basins/hydrothermal

regions (Figure 1). Biogeography is the

science that documents and explains

spatial patterns of biodiversity. The pres-

ent distribution of a species represents

(1) the historical events acting on geo-

logical time scales, or vicariance, that

have shaped its geographical range, and

(2) the dispersal potential of the species

determined by its life-history patterns,

topography, and hydrography, and acting

on ecological time scales (Tunnicliffe et

al., 1996). A number of physicochemi-

cal properties of hydrothermal vent

habitats (described above) make these

systems interesting for biogeographical

studies (Tunnicliffe et al., 1991, 1998):

(1) the species are constrained to specific,

linear habitats, (2) vent habitats are glob-

ally distributed, (3) they are discrete and

ephemeral environments, (4) there is a

high degree of endemism, (5) there are

phylogeographic relationships with other

reducing environments, such as cold

seeps on continental margins or whale

falls, and (6) identifying faunal provinces

and their boundaries allows us to rap-

idly characterize a mechanism respon-

sible for creating and/or maintaining

the province. Our present knowledge of

hydrothermal vent biology identifies six

distinct biogeographical domains char-

acterized by specific faunal assemblages:

(1) the Azores shallow Atlantic vent com-

munities (80–1700 m), (2) the fauna of

the deep Mid-Atlantic Ridge vents be-

tween the Azores Triple Junction and the

Equator, (3) the East Pacific Rise com-

munities found from 30°N to the Easter

Micro-Plate, (4) the Northeast Pacific

vent communities of the Explorer, Juan

de Fuca, and Gorda Ridges, (5) the west-

ern Pacific back-arc basin communities,

and (6) the vent fauna of the Central

Indian Ocean (Figure 1, Table 1) (Van

Dover et al., 2002; Shank, 2004).

Speciation Through Vicariance

The processes affecting biogeographic

patterns over geological time scales in-

clude vicariant events, where the move-

ment of oceanic plates plays a defining

role. Vicariance is, for example, used to

explain the formation and boundary of

the Northeast Pacific and East Pacific

Rise biogeographic provinces during the

Mid-Tertiary (~ 28 million years ago

[Ma]). Subduction of the Farallon Plate

under the North American Plate caused

the split of the Farallon-Pacific Ridge,

resulting in the separation of the Gorda-

Juan de Fuca-Explorer Ridge system,

to the north, from the East Pacific Rise,

to the south (Tunnicliffe et al., 1996).

Subsequent biological isolation permit-

Oceanography march 2007 37

ted divergent evolution of the fauna on

the now-bisected ridges, resulting in an

absence of the tubeworm Riftia pachyp-

tila, the clam Calyptogena magnifica,

and the polychaete Alvinella spp. from

Northeast Pacific vent sites, and the pro-

liferation of vestimentiferan tubeworms

on the East Pacific Rise.

Another example of a major vicari-

ant event is the closure of the Isthmus of

Panama 5 Ma and the resulting isolation

of Pacific marine fauna from the Atlantic

Ocean. Although there was a major deep-

ocean gateway in this region prior to the

closure, there were no ridge connections.

However, comparisons of cold-seep fau-

na from the Gulf of Mexico, the Oregon

margin, and the California margin indi-

cate close taxonomic similarities, suggest-

ing a dispersal pathway for seep species

between the Pacific and Atlantic Oceans

via the (now-closed) Isthmus of Panama

(Tunnicliffe et al., 1996). The only exist-

ing ridge connection between the Pacific

and the Atlantic was through a complex

mid-ocean ridge and subduction system

between South America and Antarctica,

which, in the Tertiary, could have served

as a dispersal pathway between the two

ocean basins (Tunnicliffe et al., 1996).

A similar analysis of vent fauna at the

genus level indicates a relatively close

connection between the western Pacific

back-arc and Central Indian Ridge vent

faunas as described by Desbruyères et al.,

2007. These authors suggest a potential

past dispersal pathway via the Southeast

Indian Ridge, Macquarie Ridge Complex

(south of New Zealand), and Kermadec

Arc (north of New Zealand) that would

explain these similarities. The connec-

tion via the Pacific-Antarctic ridges and

Macquarie Ridge has also been sug-

gested as a potential dispersal route for

vent siboglinid polychaetes between the

Southwest Pacific back-arc basin and the

East Pacific Rise fauna (Tunnicliffe and

Fowler, 1996; Kojima et al., 2003).

Distribution Patterns

and Ecological Processes

While tectonic dynamics affect bioge-

ography at the geological time scale, a

number of ecological and biological fac-

tors play major roles in the distribution

of species, acting within the life span of

the animal. These processes are related

to the life-history patterns of the species

and environmental factors. Marine in-

vertebrates ensure gene flow, dispersal,

and colonization through their larval

phase. The microscopic larvae of in-

vertebrates are transported passively in

the water column, and are affected by

a number of factors, both biotic (e.g.,

metabolic capacity, larval anatomy, be-

havior, swimming capabilities, and mor-

tality rate) and abiotic (e.g., deep-water

currents, geological barriers, distance

between sites, and chemical suitability of

habitats) (Cowen et al., 2000; Mullineaux

et al., 2005). The life-history traits of

marine invertebrates are very diverse,

ranging from species that produce a high

number of small eggs to species that

produce a lower number of larger eggs

(Ramirez-Llodra, 2002). Typically, lar-

vae hatching from smaller eggs develop

during a planktonic phase in which they

feed in the water column, allowing lon-

ger dispersal times but also increasing

mortality risks. In contrast, larvae hatch-

ing from larger eggs exhibit abbreviated,

lecithotrophic development, usually

feeding only on the energetic reserves

provided in the egg. These nonfeeding

larvae have, in general, lower dispersal

potentials but higher survival probabil-

ity. However, recent studies show that

the low temperature of bathyal and abys-

sal waters can result in long metabolic

life spans for larvae of vent species such

as Riftia pachyptila, allowing for long

dispersal distances (Marsh et al., 2001).

Furthermore, studies show that the em-

bryos of the vent polychaete Alvinella

pompejana can achieve long dispersal

times through developmental arrest

when dispersing in cold abyssal waters,

only completing their development upon

encountering warm water (Pradillon et

al., 2001). In hydrothermal vent species,

there are examples of all life-history pat-

terns and larval types, but the relatively

With every new discovery and investigation of

known sites and communities , our knowledge and

understanding of the diversity and functioning

of these remote and exuberant ecosystems increases ,

helping us understand the processes driving the

deep sea and the global biosphere.

Oceanography Vol. 20, No. 138

limited data on reproduction of vent

species indicates that lecithotrophic lar-

val development is dominant (Tyler and

Young, 1999; Young, 2003). It is the in-

teractions between the life-history traits

of a species and the physical, chemical,

and geological environmental factors of

the larval and juvenile habitat that shape

the dispersal and colonization of a spe-

cies; at the same time, ridge spreading

rates, deep-ocean currents, major frac-

ture zones, and other bathymetric fea-

tures can all also play important roles in

driving biogeographic patterns.

The effect of ridge spreading rate on

larval dispersal is, potentially, directly

related to the distance between active

vent sites, their life span, and the geo-

morphology of the ridge valley (see,

e.g., Baker and German, 2004). On

slow-spreading ridges such as the Mid-

Atlantic Ridge, where the time-averaged

magmatic budget is relatively low, vent

sites may be fewer and longer lived.

Consequently, the distance between po-

tentially suitable recruitment areas may

be greater, and, certainly, the presence

of large fracture zones interspersed with

numerous smaller-scale (≥ 10 km) off-

sets every few tens of kilometers along

axis creates potential physical barriers to

larval dispersal. Also, the valleys of slow-

spreading ridges have steep walls that

retain the hydrothermal plume within

the valley, potentially limiting across-

axis dispersal. In contrast, magmatically

more robust, fast-spreading ridges such

as the East Pacific Rise harbor vent sites

that may be closer in space and are cer-

tainly less likely to be offset from one

another along axis. The absence of mul-

tiple ridge-crossing fractures, in particu-

lar, creates a virtual continuum of poten-

tially favorable settlement habitats along

the ridge axis that act as stepping stones

for genetic communication among

populations (or gene flow) through

dispersal and successful colonization.

Furthermore, hydrothermal plumes are

not retained within the shallow walls

of the axial East Pacific Rise summit

graben, allowing for across-axis plume

movement and potentially greater larval

dispersal (Van Dover et al., 2002).

Large fracture zones and other topo-

graphic features (e.g., seamount chains,

depressions) can play important roles in

the gene flow of vent species; the research

community is currently addressing their

effects. A recent study on genetic dif-

ferentiation of five vent annelids—Riftia

pachyptila, Tevnia jerichonana, Oasi-

sia alvinae, Alvinella pompejana, and

Branchypolynoe symmytilida—in eastern

Pacific vents revealed barriers to dispersal

affecting one or several of these species

(Hurtado et al., 2004). The 6000-m de-

pression at Hess Deep between the East

Pacific Rise and Galápagos Rift could

limit gene flow of Alvniella pompejana

(Hurtado et al., 2004) and of the am-

phipod Ventiella sulfuris (France et al.,

1992), while the 240-km Rivera Frac-

ture Zone on the northern East Pacific

Rise may be a barrier to dispersal for

O. alvinae, T. jerichonana, and B. sym-

mytilida (Hurtado et al., 2004) as well as

for V. sulfuris (France et al., 1992).

Another controlling factor of bio-

geographic patterns may be ridges in

hydrographic isolation. An example

of a potential topographic barrier to

gene flow is found in the Arctic, where

the ultraslow-spreading ridges of the

Norwegian Sea and Arctic Basin are

isolated from the Atlantic ridge system

by the Iceland hotspot and its off-axis

trace, which generates a shallow-water

sill extending across the full breadth of

the North Atlantic Ocean. Recent geo-

chemical studies indicate the existence

of abundant active vents on the Gakkel

Ridge (Edmonds et al., 2003), but these

habitats and potential associated fauna

have not yet been directly observed. A

biological investigation of the Gakkel

Ridge during the International Polar Year

(2007–2008) will provide crucial data

for understanding the evolution of fauna

that may have thrived in isolation for the

past ~ 28 million years.

In the equatorial region of the Mid-

Atlantic Ridge, the 60-million-year-old

Chain and Romanche Fracture Zones

represent formidable geological features

(4-km high and 935 km of ridge offset)

that greatly affect both the linearity of

the ridge system and large-scale ocean

circulation in this region. It has been sug-

gested that these major facture zones re-

sult in a geological barrier to the disper-

sal of vent species along the Mid-Atlantic

Ridge axis, isolating certain species north

and south of the equator, while others,

such as the shrimp Rimicaris exoculata

whose high pelagic larval and juvenile

dispersal capacity allows it to exploit

phytodetritus in the water column, may

not be affected (Shank et al., 1998).

These hypotheses are being tested by cur-

rent research programs (see below).

Along with their potential as bar-

riers to north-south, along-axis gene

flow, the Mid-Atlantic Ridge fracture

zones may serve as a conduit for lar-

val transport through deep-water cur-

rents from west to east in the Atlantic

Ocean. The North Atlantic Deep Water

(NADW) flows south along the east

Oceanography march 2007 39

coasts of North and South America as

far as the equator before being deflected

east and crossing the Mid-Atlantic Ridge

through the fracture zones (Messias et

al., 1999). These fracture zones could be

pathways for migrants between chemo-

synthetic sites in the Gulf of Mexico to

similar habitats in the Gulf of Guinea.

Moreover, recent physical studies indi-

cate the presence of massive currents or

deep-water jets in these regions (Schmid

et al., 2005) that could increase the ve-

locity of larval transport between the

two sides of the Mid-Atlantic Ridge

and enhance the possibility of success-

ful communication among popula-

tions. It is known, for example, that the

seep-dwelling caridean shrimp Alvino-

caris muricola inhabits both Gulf of

Mexico and Gulf of Guinea cold seeps

(Ramirez-Llodra and Segonzac, 2006);

in addition, the siboglinid tubeworm

Escarpia southwardae has recently been

described at the Gulf of Guinea seeps,

with morphological and genetic charac-

teristics that indicate a close taxonomic

relationship with E. laminata from the

Gulf of Mexico seeps (Andersen et al.,

2004). While these findings may suggest

a certain degree of gene flow across the

Mid-Atlantic Ridge, data are still very

limited. Further molecular and larval

ecology studies are needed in order to

fully understand dispersal pathways of

vent species, and particular attention

should be paid to the effect of larval re-

tention by local hydrodynamics (Marsh

et al., 2001; Mullineaux et al., 2005) and

the decrease in larval concentration by

diffusion and mortality (Cowen et al.,

2000). In the southern oceans, the Cir-

cumpolar Current has been suggested as

a pathway that may link species from the

southern East Pacific Rise, the East Scotia

Rise near Antarctica, the southern Mid-

Atlantic Ridge, and the Southwest Indian

Ridge (Figure 1) (Van Dover et al., 2002;

Hurtado et al., 2004).

Finally, depth may also play a signifi-

cant role in biogeographic patterns, as

suggested by Desbruyères et al. (2001)

for the differences found near the Azores

Triple Junction, where depth decreases

from 2400 m at Rainbow to 850 m at

Menez Gwen. This depth variation

causes changes in fluid toxicity and sus-

pended mineral particles, accompanied

by an impoverishment of vent endemic

species at shallower depths and an in-

crease in non-vent bathyal species. These

authors also suggest that the existence of

several biogeographic islands in this re-

gion is driven by depth.

lookiNG toWarD the FutureThirty years after the discovery of hy-

drothermal vents, the investigation of

vent habitats and their associated fauna

is still in a critical exploratory and dis-

covery phase. Every new systematic

survey of a ridge section uncovers new

vent sites and their biological commu-

nities, often yielding species new to sci-

ence and sometimes new physiological

or morphological adaptations. The latest

discoveries span from pole to pole and

around the globe, including the detec-

tion of hydrothermal plume signals in

both the Arctic (Edmonds et al., 2003)

and Antarctic regions (German et al.,

2000; Klinkhammer et al., 2001), the dis-

covery of new hydrothermal sites along

the Central Indian Ridge (Van Dover

et al., 2001), and the first investiga-

tions of vent sites on the Mid-Atlantic

Ridge south of the equator (German

et al., 2005; Shank, 2006). The geologi-

cal, geochemical, physical, and biologi-

cal investigations of these regions will

provide essential information toward

completing the biogeographical puzzle

of vent biogeography (Figure 1), increas-

ing our knowledge of species diversity

and distribution, and providing the nec-

essary clues to understand what factors

drive and shape vent communities and

species distribution.

To address the gaps in our knowledge

of biogeography in deep-water chemo-

synthetic ecosystems (including hydro-

thermal vents, cold seeps, whale falls, and

regions of low oxygen and other reduc-

ing habitats), the international scien-

tific community is joining efforts in the

framework of the ChEss program (www.

noc.soton.ac.uk/chess). ChEss is one of

the 14 field projects of the Census of

Marine Life (www.coml.org), a 10-year

global initiative to describe the diver-

sity, abundance, and distribution of life

in the oceans. Within this context, the

main goal of ChEss is to describe the

biogeography of species from deep-water

chemosynthetic ecosystems and under-

stand the forces driving them (Tyler et

al., 2003). To achieve this goal, ChEss

has selected a number of priority study

areas based on our present knowledge of

biogeography (Figure 1). These include:

(1) the Atlantic Equatorial Belt, to ad-

dress scientific questions of gene flow

across and along the Mid-Atlantic Ridge

equatorial fracture zones; (2) the South-

east Pacific region off Chile, a unique

site where the Chile Ridge is subducting

beneath the South American Plate and

where we find all known chemosyn-

thetic habitat types in close proximity;

Oceanography Vol. 20, No. 140

(3) the New Zealand region, where vents,

seeps, whale falls, and other reducing

ecosystems are also found in close prox-

imity; and (4) the polar regions, with

development of field programs along the

potentially isolated Gakkel Ridge and in

the Antarctic. Large-scale national and

international collaborations and sharing

of human and infrastructure resources

are making such research programs pos-

sible, greatly facilitated by coordination

of international programs such as ChEss

and InterRidge (www.interridge.org).

The exploration, investigation, and

sampling of remote, dynamic, and

topographically complex ecosystems

such as hydrothermal vents also requires

continuing development of state-of-

the-art technologies for fieldwork and

laboratory analyses. For example, to

continue exploration at ever-higher lati-

tudes, robotic vehicles are increasingly

important—both remotely operated

vehicles and autonomous underwater

vehicles (AUVs). Indeed, with so much

of the global mid-ocean ridge yet to be

investigated, AUVs that can operated

independently of a mother ship are in-

creasingly recognized as one technology,

in particular, that will be vital to push

back the barriers to our knowledge (see,

e.g., Yoerger et al., this issue).

coNcluSioNExploration and investigation of new

sites at key locations is essential to fill

in important gaps in the biogeographi-

cal puzzle of hydrothermal vents. The

existence of such ecosystems was com-

pletely unexpected 30 years ago, but

their discovery has changed the way we

understand both Earth and the life upon

it. With every new discovery and inves-

tigation of known sites and communi-

ties, our knowledge and understand-

ing of the diversity and functioning of

these remote and exuberant ecosystems

increases, helping us understand the

processes driving the deep sea and the

global biosphere. Continuing explo-

ration of hydrothermal vents will no

doubt lead to the description of new

species, a better understanding of geo-

logical and geochemical processes affect-

ing their biology (and vice versa), and

the understanding of the phylogenetic

links between vent species and other

faunal communities. As an important

societal benefit, this research will also

undoubtedly continue to provide po-

tentially interesting sources of active

molecules for the biotechnological and

biomedical industries.

ackNoWleDGemeNtSE. Ramirez-Llodra is supported by

the ChEss-Census of Marine Life pro-

gram (A.P. Sloan Foundation), which

is kindly acknowledged. C.R. German

also acknowledges support from ChEss-

Census of Marine Life and further sup-

port from the Natural Environment

Research Council (UK) and from the

US National Science Foundation (NSF)

and National Oceanic and Atmospheric

Administration (NOAA). T. Shank

acknowledges support from NSF, the US

National Aeronautic and Space Admin-

istration Astrobiology Program, NOAA-

Ocean Exploration, and the Deep-Ocean

Exploration Institute at the Woods Hole

Oceanographic Institution. The authors

thank Paul Tyler, Chuck Fisher, Kristen

Kusek, and an anonymous reviewer for

their comments and suggestions on an

earlier version of this manuscript.

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